1. Field of the Invention
The present invention relates to a method and a device for coating a hollow cylindrical member.
2. Description of the Related Art
In a typical image forming apparatus such as a copier, a facsimile, or an LBP (laser beam printer) that uses electrophotography to form an image, a toner image transferred to transfer paper is melted on and fixed thereto when the transfer paper passes between a heated fixing member and a pressure member pressing against the fixing member.
The fixing member used in image fixing, such as a fixing roller or a fixing belt, typically includes a hollow cylindrical core formed of a metal such as aluminum or iron or an endless substrate formed of a resin such as polyimide or a metal such as nickel. The core or the substrate is coated with a heat-resistant elastic layer formed of silicone rubber and may then be tubularly covered or coated with a releasing layer formed of fluororesin.
The elastic layer functions to evenly press the toner against the transfer paper during image fixing, thereby reducing image graininess. In addition, the heat conductivity of the elastic layer affects a device startup time (i.e. the time required for the temperature to rise to a certain level). Therefore, it is desirable that the thickness of the elastic layer be uniform.
In a fixing device using such a fixing member, electrical conducting properties are imparted to the fixing member to improve fixing ability and prevent image density irregularities. Further, when a fixing belt is used as the fixing member with a supporting member contacting the inner surface of the fixing belt for structural reasons, tribological properties are imparted to the inner surface of the fixing belt so that the fixing belt is steadily driven.
A substrate included in the fixing member may have such properties. However, a substrate such as a metal substrate has poor tribological properties and therefore a coating material having the above-described properties is applied to the inner surface of the substrate.
However, in this case, processes of forming and drying the inner and outer coatings are sequentially performed, thereby increasing the man-hours required and the cost incurred. In addition, for the processes to be sequentially performed, the substrate, which is a hollow cylindrical member, needs to be positioned or held with high accuracy in different ways each time for internal coating and external coating, which makes the coating system expensive.
To solve the problems described above, development of a method for simultaneously coating the inner and outer surfaces of a hollow cylindrical member is proceeding.
The inner and outer coatings of the hollow cylindrical member often need to be of different materials and different thicknesses to have different properties as described above. Therefore, it is desirable that the inner and outer surfaces of the hollow cylindrical member be simultaneously coated with different coating materials. Consequently, a typical dip coating method in which the hollow cylindrical member is dipped in and out of a large coating tank is too simple to solve the above-described problems.
To facilitate an understanding of the state of the art and of the present invention typical methods known for coating either the outer or the inner surface of a hollow cylindrical member are described below.
The methods of coating the outer surface of a hollow cylindrical member include a dip coating method, a ring coating method, an annular curtain coating method, a spray coating method, a blade coating method, and a roller coating method. In addition to the above-described methods, the methods of coating the inner surface of a hollow cylindrical member include a coating method in which a coating material is discharged onto the inner surface of a rapidly rotating hollow cylindrical member so that the coating material is centrifugally pressed against the inner surface, and a coating method in which the inner surface of the hollow cylindrical member physically contacts the primarily coated outer surface of a coating pipe inserted into the hollow cylindrical member.
The inner and outer surfaces may be simultaneously coated by combining the coating methods described above. However, such coating causes a lack of uniformity in the thickness of the coating.
Typically, to form a flat, smooth, and uniform coating on the outer surface of a hollow cylindrical member by the ring coating method, there needs to be a constant gap CG (coating gap) between a slit of an annular coating head and the hollow cylindrical member, i.e. the hollow cylindrical member needs to be concentric with the annular coating head, over the entire coating area in the axial direction of the hollow cylindrical member.
Specifically, when the hollow cylindrical member is not concentric with the annular coating head, it is difficult to form a uniform and highly accurate coating over the entire coating area. The thickness of the coating varies along the circumferential direction of the hollow cylindrical member, and an uneven circumferential shear force is applied to the hollow cylindrical member and causes small bubbles or vertical streaks to form in the coating. Therefore, the hollow cylindrical member needs to be positioned with high accuracy to be coaxial with the annular coating head. However, such coaxial arrangement cannot be achieved with a flexible substrate such as an endless belt since simultaneous coating of the inner and outer surfaces does not allow the use of a core, which is effective for configuring the coaxial arrangement, on the inner surface of the flexible substrate.
The coaxial arrangement is also required during movement of the annular coating head relative to the hollow cylindrical member. Therefore, the annular coating head needs to be driven vertically with high accuracy, which increases the device cost.